Vortex liquid entanglement in irradiated YBa_2Cu_3O_7 thin films

Resumen

Epitaxial YBa_2Cu_3O_7 thin films, grown by high-pressure de sputtering, an irradiated with He^(+) ions at 80 keV with doses between 10_(14) and 10_(15) cm^(-2). Irradiation reduces the critical temperature but it does not modify the carrier concentration. Angle- dependent resistivity is used to show that the mass anisotropy does not change upon irradiation. The melting transition in magnetic fields applied parallel to the c axis is analyzed by I-V critical scaling, and all irradiated and nonirradiated samples show a three- dimensional vortex glass transition with the same critical exponents. The dissipation in the liquid state is analyzed in terms of the activation energy of the magnetoresistance in a perpendicular magnetic field. While as-grown samples show an activation energy depending as 1/H on the applied magnetic field, irradiated samples show a dependence as 1/H^(0.5), characteristic of plastic deformation of vortices. This is discussed in terms of the point disorder introduced by ion irradiation.